msm: mdss: reduce log level when ping pong timeout happens

In the current implementation, driver has a recovery mechanism
from pingpong timeout on command mode interface. This change
replaces WARN macro with pr_warn to reduce the logging in dmesg
when the pingpong timeout happens only once.

CRs-Fixed: 856973
Change-Id: I7568466f4e909e4adafaabbcd5da22596750914b
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>